1. Field of the Invention
The invention pertains to a method for operating an inkjet printing apparatus with at least one inkjet printhead having a row of nozzles for directing ink onto a substrate being conveyed on a curved guide element in a transport direction.
2. Description of the Related Art
In printing presses which operate according to the offset printing principle, especially web-fed rotary presses and sheet-fed presses, increasing use is being made of inkjet printing devices, which serve primarily to individualize the printed materials produced by offset printing by adding to them, for example, barcodes, numbering, or other types of labeling. These inkjet printing devices have at least one inkjet printhead, which can be designed according to the so-called “continuous” inkjet principle, the drop-on-demand inkjet principle, the thermal inkjet principle, the bubble inkjet principle, or any of the other inkjet principles. The inkjet printheads usually have a row of nozzles consisting of several adjacent nozzles, through which the ink can be directed onto the substrate to be printed.
In many applications of inkjet printing devices, it is necessary to use a large number of inkjet printheads, some arranged transversely to the transport direction of the substrate, i.e., transversely to the printing direction, and others arranged in the printing direction. The required number of inkjet printheads transverse to the printing direction is defined primarily by the desired print resolution in relation to the given print resolution of the selected inkjet printhead and by the desired overall printing width relative to the printing width of an inkjet printhead. The required number of inkjet printheads in the printing direction is determined primarily by two factors: first, the fact that the desired printing speed is greater than the given printing speed of an inkjet printhead, and, second, the fact that several printing inks are to be applied to the substrate by the inkjet printing device.
This can lead to the situation that a large number of inkjet printheads in the inkjet printing device must be arranged next to each other in an array-like or matrix-like manner both transversely to the printing direction and in the printing direction.
Regardless of whether the inkjet printing device used to print the substrate has several inkjet printheads arranged in an array or only a single inkjet printhead, the print resolution transverse to the printing direction is determined primarily by the distance between the nozzles adjacent to each other in the row of the inkjet printhead. To increase the print resolution despite this limitation, it is already known according to the prior art that the inkjet printheads of an inkjet printing device can be oriented at a slant to the transport direction of the substrate and thus to the printing direction. The result is that the effective distance between the nozzles transverse to the printing direction or transport direction of the substrate is reduced, which means that the print resolution can be increased.
If an inkjet printing device of this type with printheads which have rows of nozzles arranged at a slant to the printing or transport direction of the substrate is used to print the substrate as it is being guided over a cylinder or over a curved surface, the problem occurs that the printed image can be distorted. So far there are no approaches known according to the prior art which can be taken to avoid these types of distortions.